SWR (non-super) Redhead.... help?

Anticpunk · 13th February 2011, 09:02 AM

Hello,

I was hoping maybe somebody around here can shed some light on the power section of the SWR Redhead, the non-Super variety.

Scenario:

I got this in as a DOA. Melted fuse holder, 4 bad main caps, 2 burnt 2SD424s (of 4), 3 cooked resistors, and the 3 or 4 small electrolytics on the PA board. Everything that looked, smelled and tested funky got replaced. I couldn't find an immediate source for REAL 2SD424s, so I used 4 new 2N3773 as substitutes. Specs seem within the right range, so in they went.

So Today I get it buttoned back up, put juice to it, tube warmed up. No Problemo, or so it would seem.

I had a little low volume distortion at first, but I figured since it went away almost immediately, that it was nothing. Turned the amp up and it seemed a little dim for being "240 watts". After about 15 minutes of idle and light playing the heat sink was warm, but not hot, transformer was cold, preamp voltage and rail voltage checked out. played it some more, digging in a little bit this time. At roughly the 25minute mark it just clicked off.

Pulling the chassis, I found the inline leaded fuse had blown gloriously, but thats the extent of new damage.

Anybody have any insight into a possible why? If the transistor bias was off, wouldn't the transistors be super hot? if not, maybe some indicative smoke of some kind at least? Is it possible that I roasted the new OP transistors already?

Admittedly, I have basically no experience with transistor bias or the set procedure, and I try to steer clear of working on SS amplifiers, (basically due to these kinds of frustrations in troubleshooting), but I've been messing with tube amps for awhile and am guessing its the same basic theory. The SWR has a trimpot which the schematic, (for the Super Redhead unfortunately), that fender sent me shows but doesn't explain further.

I've read a lot of posts, here there and everywhere, and I'm not finding what I seek.

I thank you in advance for the help.

-Jay

Tarzan · 14th February 2011, 07:30 AM

Can you measure the current it takes when idle?
No load and no input signal present.
Insert an amp meter (range 200mA) in to the plus (+) power supply line to start with.
Should be no more then 50..60 mA
Adjustable with the bias trimmer.

Come back with the readings.

13th February 2011, 09:02 AM	#1
Anticpunk diyAudio Member Join Date: Feb 2011	SWR (non-super) Redhead.... help? Hello, I was hoping maybe somebody around here can shed some light on the power section of the SWR Redhead, the non-Super variety. Scenario: I got this in as a DOA. Melted fuse holder, 4 bad main caps, 2 burnt 2SD424s (of 4), 3 cooked resistors, and the 3 or 4 small electrolytics on the PA board. Everything that looked, smelled and tested funky got replaced. I couldn't find an immediate source for REAL 2SD424s, so I used 4 new 2N3773 as substitutes. Specs seem within the right range, so in they went. So Today I get it buttoned back up, put juice to it, tube warmed up. No Problemo, or so it would seem. I had a little low volume distortion at first, but I figured since it went away almost immediately, that it was nothing. Turned the amp up and it seemed a little dim for being "240 watts". After about 15 minutes of idle and light playing the heat sink was warm, but not hot, transformer was cold, preamp voltage and rail voltage checked out. played it some more, digging in a little bit this time. At roughly the 25minute mark it just clicked off. Pulling the chassis, I found the inline leaded fuse had blown gloriously, but thats the extent of new damage. Anybody have any insight into a possible why? If the transistor bias was off, wouldn't the transistors be super hot? if not, maybe some indicative smoke of some kind at least? Is it possible that I roasted the new OP transistors already? Admittedly, I have basically no experience with transistor bias or the set procedure, and I try to steer clear of working on SS amplifiers, (basically due to these kinds of frustrations in troubleshooting), but I've been messing with tube amps for awhile and am guessing its the same basic theory. The SWR has a trimpot which the schematic, (for the Super Redhead unfortunately), that fender sent me shows but doesn't explain further. I've read a lot of posts, here there and everywhere, and I'm not finding what I seek. I thank you in advance for the help. -Jay

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14th February 2011, 07:30 AM	#2
Tarzan diyAudio Member Join Date: May 2004 Location: Genk	Can you measure the current it takes when idle? No load and no input signal present. Insert an amp meter (range 200mA) in to the plus (+) power supply line to start with. Should be no more then 50..60 mA Adjustable with the bias trimmer. Come back with the readings.

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